Nitrogenous compounds are a component of waste products generated by finfish and shellfish aquaculture. Ammonia and nitrite are toxic to finfish and shellfish. Removing ammonia and nitrite is essential, particularly for recirculating aquaculture systems. Biofilters remove toxic nitrogenous compounds by exposing aquaculture water to media colonized with nitrifying bacteria which oxidize ammonia and nitrite to less toxic nitrogenous compounds such as nitrate.
Micro bead biofilter (MBBF) systems filter water through a media bed comprising buoyant microbeads coated with a biofilm of nitrifying bacteria. Sufficient mixing of the media involves causing the microbeads to continuously move from a top of the media bed to the bottom of the media bed and then back to the top of the media bed again. Hydraulic loading rate refers to the rate of water flow required for sufficient mixing to occur. Sufficient mixing is necessary for proper nitrification and aeration, and to prevent clogging and excess biomass buildup.
In typical MBBF systems media beds are relatively shallow due to bed depth being limited by the necessity for sufficient media mixing and, in turn, limited by the energy available for media mixing. Energy is provided solely by gravity flow of water, which satisfies hydraulic loading rate requirements for media beds contained in chambers only up to about 18″ (45 cm) deep. Given limited media bed depth, increasing nitrification capacity of known MBBF systems requires increasing the footprint of the media bed, resulting in greater capital costs. Media beds of known MBBF systems also tend to “bunch” near the bottom of the chambers due to the downward gravity flow of water; this bunching lowers nitrification capacity and interferes with water flow through the chambers.
In addition to ensuring proper nitrification, lowering energy costs is another significant issue in aquaculture operations. Energy costs are a major factor in the profitability of recirculating aquaculture systems. Electricity for meeting the pumping requirements of recirculating aquaculture systems represents a significant portion of these energy costs.
Efficient and cost-effective biofilters and recirculating aquaculture systems are desirable.